This invention relates to protection of heat exchanger tube ends.
Heat exchangers often contain a large number of metal tubes through which heat exchanging fluid flows. The tubes are usually arrayed in parallelism and are supported adjacent their ends by transverse tube sheets. See, for example, U.S. Pat. No. 4,489,776 issued Dec. 25, 1984. The tube ends are disposed in the heat exchanger heads which form chambers for the containment and transfer of fluid from and to the tubes.
Heretofore it has been observed that, after long periods of service, the inner surfaces of the tube end portions, and especially the inlets, often tend to become heavily pitted or worn away, thus weakening the entire structure. It is believed that such undesirable effects are due to erosion, and also in some instances corrosion, of the metal pipe surface, which may extend far back into the tubes. The erosion is believed due to the existence of turbulence of the fluid for a finite distance inwardly of the tube ends, combined with the eroding action of silt or other contaminants in the fluid. The turbulence ends at a point inwardly of the tube ends so that the fluid flows in a laminar fashion throughout the remainder, and longest, portion of the tubes. In the areas of laminar fluid flow, there is substantially less turbulence or erosion. Furthermore, metals tend to corrode due to chemical action of the fluid. The combination of erosion and corrosion of the eroding tube end portions thus leads to weakened tubes.
Previous attempts have been made to protect heat exchanger tube end portions from the deleterious effects of the turbulence-caused erosion, together with corrosion, which occurs and is visually observable as a damaged tube section on the inner surface thereof for a finite distance inwardly of the tube ends. One such attempt has been to provide metallic inserts which are fit, rolled and sometimes welded into the tube ends for the purpose of preventing erosion and damage to the end portions. The inserts are of a length to cover the damaged tube sections. However, the use of such metal inserts has not solved the problem and has given rise to further problems.
The prior metal inserts are also subject to erosion and corrosion, and in effect become sacrificial elements which themselves are damaged over long periods of use. When such inserts become damaged, they need to be replaced with new inserts, but it has been found that the interfaces between the inserts and tubes have often become corroded by galvanic action so that the inserts cannot be removed without destroying the tube ends.
Yet another difficulty occurs because the metal inserts reduce the effective internal diameter (I.D.) of the tubes. In the event that it is desirable to retrofit the heat exchanger with sets of shuttleable tube cleaning elements such as brushes which are captured by cages or baskets disposed at each end of the tubes, the brushes must be made smaller than the I.D. of the tubes in order to pass through the reduced I.D. of the inserts. By itself, this difference in brush diameter might not adversely affect their cleaning ability. However, most brush capturing baskets are provided with short necks which are fit into the tube ends. When the tube ends include I.D. reducing inserts, the basket necks further reduce the effective I.D. and thus the available diameter for the brushes to pass through. Thus, the brushes must be made substantially smaller in diameter than the brushes designed for the original tube I.D. The result is that the smaller brushes do not firmly contact the walls of the tubes during brush shuttling, impairing their cleaning function. Furthermore, baskets originally designed for a given tube I.D. will have to be redesigned to handle an effective tube end of smaller diameter.
It is a task of the present invention to solve the various problems discussed above so that the finite areas of fluid turbulence adjacent the tube ends of a heat exchanger or the like are accompanied by substantially less or no erosion or corrosion, as compared to prior known systems. It is a further task to substantially eliminate the double reduction in tube I.D. of the above-described prior systems so that when a tube cleaning brush and basket system is utilized in conjunction with an insert, the brush I.D.s do not need to be reduced by an amount which adversely affects the cleaning function thereof. It is yet another task to combine the various elements to provide a brush passage of essentially a single diameter in the areas of fluid turbulence at the ends of heat exchanger tubes.
In accordance with the various aspects of the invention, a basically non-corrodible elongated sleeve-like insert is provided, with the insert also being erosion resistant. These desirable characteristics are accomplished by making the insert of non-metallic material such as sythetic rubber or plastic. The insert's outside diameter (O.D.) is closely similar to the I.D. of the heat exchanger tubes so that the insert may be inserted and removed from the tube end with relative ease. The insert length is intended to correspond closely with the length of the finite tube end section subject to damage from fluid turbulence, which has been found to be in the range of about 5 to 12 inches depending upon the tube I.D. as well as fluid velocity and pressures. If desired, and to prevent undesirable penetration of corrosive materials and gases into any void that may exist between the non-metallic insert and metallic heat exchanger tube, a filler of epoxy or the like may be applied to fill the void.
In accordance with further inventive aspects, the insert is contemplated as providing only a single element reduction in passage diameter. When a tube cleaning brush and basket system is also utilized, brush diameter reduction relative to the tube I.D. is minimized.
In connection with a tube cleaning brush and basket system, the outer insert end may conveniently serve as the connection to a brush capturing plastic basket. In the embodiments disclosed herein, the insert and basket are connected via cooperating thread means, or may be connected integrally. In both cases, the need for a tube I.D.-reducing press fit of the basket is eliminated and the basket need not be reduced in diameter relative to the heat exchanger tube. The insert and basket together form a continuous brush-receiving channel leading to the open cage portion of the basket.